1. Field of the Invention
This invention relates to an image bar having an array of thermally activated dot shutters or light valves, and more particularly, to an image bar having a pair of confronting prisms containing a layer of liquid and an array of transparent heating elements therebetween. This liquid has an index of refraction substantially matched to that of the prisms, so that light passing through the prisms may be re-directed by total internal reflection upon the selective activation of the heating elements which produce temporary bubbles that displaces the liquid from the critical optical interface.
2. Description of the Prior Art
U.S. Pat. No. 3,730,608 to Castegnier discloses a light modulation system employing the use of an electrolytic substance. With reference to FIG. 4, when electrode 10 is electrically energized, a gas medium 8 is formed in electrolyte 2 that contacts with the face of prism 16. When light beam 19 enters into the system, it is totally reflected at the interface of gas medium 8. When electrode 10 is not energized, gas medium 8 disperses and light beam 9 passes through the electrolyte without substantial reflection.
U.S. Pat. No. 4,148,563 to Herbert discloses a panel which may be either transparent or reflective. As shown in FIG. 1, the panel is composed of two mated transparent layers, the confronting surfaces of which have V-shaped ridges and a film of liquid therebetween. The index refraction of the liquid is nearly that of the layers so that an incident ray of light will be transmitted therethrough. If the fluid is vaporized the incoming light will be reflected. Thus, whether the panel is transmissive or reflective, depends upon the temperature of the fluid sandwiched between the mated layers.
An article entitled "A Liquid-Vapor Display" by George W. Taylor, Proceedings of the IEEE, Vol. 61, No. 2, February 1973, discloses a reflective display having a roughened glass surface wetted with a transparent volatile liquid of the same refractive index as the glass panels sandwiching the liquid. The electrical heating via transparent electrodes of a thin film of liquid adjacent to the roughened surface transforms the glass into a light scattering surface. Referring to FIG. 1, incoming light will pass through the display to the black background. However, when the transparent electrodes are energized, vapor bubbles form around the roughened surface. This causes the incoming light to scatter before reaching the black background.
An article entitled "Liquid Evaporation Light Deflector", by George W. Taylor, Applied Optics, Vol. 11, No. 3, March 1972, discloses a digital light deflector which is based on the electrically controlled vaporization of a high refractive index transparent liquid at an inclined interface formed by the liquid in a transparent solid. Deflectors operating on this principle can be designed to provide multiple position linear or angular displacement of a light beam in one or two dimensions. Such deflectors can be random-accessed, are capable of large angular deflection, occupy a small volume, and should have a long operating life. Millisecond deflection is possible at power levels of 0.3 W/mm.sup.2 of light beam area. The deflectors have high optical transmission efficiency with on/off contrast ratios of 150:1 and higher.
An article entitled "Fully Optical Switch Looms for Video", Electronics, Feb. 10, 1986, discusses a liquid dielectric optical switch by Thomson-CSF Central Research Laboratory. A liquid dielectric optical switch referred to as "Lidos", is based on the reflection of light within a prism and electrically induced motion of a dielectric liquid. The switch consists of two prisms with their hypotenuses parallel at a distance of from 10 to 30 microns. Each hypotenuse surface has a pair of transparent electrodes. Optical fibers are coupled to three of the four sides of the prisms by way of lenses, each aimed at the center electrode on its respective prism. One fiber is input, the others output. Movement of a droplet of liquid dielectric from one set of facing electrodes to the other controls the direction of the output signal. When the space between the two center electrodes is empty, internal reflection at the hypotenuse of the prism switches light entering one side of a prism at a 90 degree angle toward the other surface of the same prism for injection into its respective output fiber. When the space between the two center electrodes is occupied by the liquid dielectric, the refractive property of the prism's hypotenuse is cancelled and the light passes through the facing prism into the opposite output fiber.
U.S. Pat. No. 3,623,795 to George W. Taylor discloses an electro-optical system embodying a material whose optical properties change sharply in a small temperature range. The material is heat biased to a temperature in or close to this range and a beam of light is directed at the material. In response to a signal applied to the material, its temperature is changed through a relatively small range, such as from a value on one side to a value on the other side of a certain critical temperature and the change in optical properties of the material which results sharply changes a characteristic such as deflection angle, polarization direction, or other parameter of the light beam.
U.S. Pat. No. 3,303,332 to George W. Taylor discloses a light modulator and display device which can be switched between a light scattering and non-light scattering condition comprising a volatile liquid in a container, said container having a front viewing face of a transparent material with a thin transparent electrically resistive coating thereon. This coating is used for heating the volatile liquid in the container to its vaporization point. The surface of the coating in contact with the liquid is highly non-planar, that is, rough, so as to provide a multiplicity of nucleation points for a vaporization of the volatile liquid and also to provide a geometry for light scattering.
U.S. Pat. No. 3,612,653 to Jan A. Rajchman discloses a digital light deflector which includes an electrically conductive transparent film deposited on transparent substrate. The transparent film acts as a heating element and is immersed in a transparent liquid in a transparent container. Light is directed through the container to the film at an angle therewith. The light normally continues in a straight line through the film, the liquid and out one side of the container. When an electric current is applied to the film, heat is generated which vaporizes the liquid at the surface of the film and causes the light to be reflected by the film vapor interface and passes out through a different side of the container.
Japanese Patent Application No. 57-128566, filed July 23, 1982, and published without examination on Jan. 31, 1984 under Laid Open No. 59-18930 and assigned to Canon Incorporated discloses method and apparatus for modulating light by generating bubbles in a liquid in response to input signals which heat transparent or reflective heating elements to modulate incident light on the device by the bubbles.
An article entitled "Liquid Thermo-Optic Modulator" by K. Minoura et al, The 13th Congress of the International Commission for Optics, held Aug. 20-24, 1984, Sapporo, Japan, discloses a liquid thermo-optic modulator essentially comprising a liquid and a thin film heater. The liquid is enclosed with a transparent cover and a substrate with a heater. The electric current applied to the heater causes a gradient index distribution in the liquid. This index of refraction change by the temperature change in the liquid causes the incident light normally reflected from the heating element to be deformed. The non-deformed light is blocked by a stop and only the deformed light passes by the stop in a Schlieren optical system.
U.S. application Ser. No. 823,977, filed Jan. 30, 1986, entitled "Thermo-Optic Light Modulation Array", to Gerald Domoto et al, and assigned to the same assignee as the present invention, discloses an optical modulator wherein collimated light from a light source is passed through a lens system to produce and direct a sheet of light towards a photoconductive member of an electrophotographic printer. In one embodiment, a transparent medium is mounted over and in intimate contact with a linear array of heating elements. The transparent medium and heating element array are positioned between the lens system and the photoconductive member so that the sheet of light passes through the transparent medium and very close to the heating element. The heating elements are substantially parallel to the sheet of light and perpendicular to the direction of the sheet of light. Means are provided to energize selected heating elements for predetermined time periods in response to digitized data signals, in order to apply bursts of thermal energy to regions of the transparent medium in the immediate vicinity of the energized heating element. The burst of thermal energy temporarily changes the refractive index of the transparent medium for a relatively short time to refract a segment of a sheet of light passing through the transparent medium. A stop blocks all modulated portions of the sheet of light, while the unmodulated portions of the sheet of light continuously impinge upon a moving precharged photoconductive member. As is well known in electrophotography, electrostatic latent images are formed by exposing a precharged photoconductive member to light which discharges or erases the background areas of the image. Several embodiments are disclosed, and in one, the sheet of light enters the transparent medium and reflects off of the heating elements, exits the transparent medium in a direction to pass by or through a stop, and then impinges on the photoconductive member. Selective energization of the heating elements modulate the light. In an alternate embodiment of the reflective type, the heating element array may be two dimensional. In this case, the light beam has sufficient thickness to project a two dimensional image when the appropriate heating elements are energized. In another embodiment, the sheet of light passes through a transparent linear array of heating elements normal to the direction of light. The heating elements surround the surface of the transparent medium so that the unmodulated light impinges on the photoconductive member, selective energization of the heating elements prevents the portion of the light passing through the heating element from striking the photoconductive member. The transparent electrode is preferably configured in a grating pattern.
U.S. application Ser. No. 756,424, entitled "Thermo-Optic Light Valve", filed July 18, 1985, to Doyle Skinner, Jr., also assigned to the same assignee as the present invention, discloses a light modulating device comprising parallel glass substrates sandwiching therebetween an array of transparent electrodes and an opaque liquid. Selectively addressed heating elements form bubbles which push the opaque liquid aside, permitting light to pass therethrough.
When considered for use as an image bar for an electrophotographic printer, the above prior art suffers one or more disadvantages such as, for example, the requirement of complex optical systems, relatively slow imaging speed, low resolution, and shorter operating lifetimes than desired because of heating element failure or heating element to electrode interface failure. These shortcomings are overcome and other advantages achieved by the present invention.